一對多映射 Hibernate

Question

我試圖在資料庫的 Comment 表中使用帖子 id 的外鍵保留 6 條評論，但最後 3 條評論使用新添加的外鍵覆蓋了前 3 條評論。

測試類別：

Comments comments = new Comments("1st Comment", new Date(System.currentTimeMillis()));
Comments comments2 = new Comments("2st Comment", new Date(System.currentTimeMillis()));
Comments comments3 = new Comments("3st Comment", new Date(System.currentTimeMillis()));

ArrayList<Comments> arrayList = new ArrayList<>();
arrayList.add(comments);
arrayList.add(comments2);
arrayList.add(comments3);

// Insert Without Comment
Post post1 = new Post("1st Post", "1st Post Description", new ArrayList<Comments>(), new Date(System.currentTimeMillis()));
postReposetory.save(post1);

// Insert With Comment
Post post2 = new Post("2st Post", "2st Post Description", arrayList, new Date(System.currentTimeMillis()));
postReposetory.save(post2);

// Update (Insert Comment)
post1.setComments(arrayList);
post1.setUpdatedAt(new Date(System.currentTimeMillis()));
postReposetory.save(post1);

Answer 1

您總共建立了 3 個評論實例（因此資料庫表中有 3 筆記錄），而不是每個貼文 3 個實例。

當您更新 post1 評論時，您會將 post2 評論作為參數，因此從 comments 到 post2 的外鍵將變更為 post1。

如果您希望每個貼文有 3 則評論，則總共需要 6 個評論實例（2 個貼文 * 3 則評論）。

Answer 2

發生這種情況是因為您放置了相同的註釋對象，然後 hibernate 認為您想要將註釋的連接從 post2 更改為 post1。

因此您必須重新建構這三個註解。

    Comments comments = new Comments("1st Comment", new Date(System.currentTimeMillis()));
    Comments comments2 = new Comments("2st Comment", new Date(System.currentTimeMillis()));
    Comments comments3 = new Comments("3st Comment", new Date(System.currentTimeMillis()));

    ArrayList arrayList = new ArrayList();
    arrayList.add(comments);
    arrayList.add(comments2);
    arrayList.add(comments3);

    // Insert With Comment
    Post post1 = new Post("1st Post", "1st Post Description", new ArrayList(), new Date(System.currentTimeMillis()));
    postReposetory.save(post1);
    
    // Insert Without Comment
    Post post2 = new Post("2st Post", "2st Post Description", arrayList, new Date(System.currentTimeMillis()));
    postReposetory.save(post2);

    // Update (Insert Comment)
    comments = new Comments("1st Comment", new Date(System.currentTimeMillis()));
    comments2 = new Comments("2st Comment", new Date(System.currentTimeMillis()));
    comments3 = new Comments("3st Comment", new Date(System.currentTimeMillis()));
    post1.setComments(List.of(comments, comments2, comments3));
    post1.setUpdatedAt(new Date(System.currentTimeMillis()));
    postReposetory.save(post1);

這樣就創建了另外三個物件用於評論。